An automotive drive battery (or, rather, ‘battery pack’) comprises a number of electrochemical cells, which are used to convert accumulated chemical energy into electric energy, and are series-connected to sum the voltage of the individual cells. In other words, series-connecting the cells to the battery terminals produces a high enough voltage for the battery to supply considerable electric power, while still maintaining acceptable electric current intensity. For example, a typical hybrid-vehicle drive battery has a rated voltage of 400 volts at its terminals, and comprises a hundred series-connected cells, each with a rated voltage of 4 volts.
Because of inevitable differences in the manufacturing tolerances and dispersion of the battery cell component parts, the capacity of the cells (i.e. the amount of electric charge that can be supplied, and normally expressed in ampere-hours) differs significantly—by roughly ±10 percent of the rated value in currently marketed batteries. And because the cells are series-connected, the capacity of the battery (i.e. of the series-connected cells as a whole) is limited to the lowest capacity (i.e. to the capacity of the cell with the lowest capacity). In other words, only the least-performing cell is used to the full. In currently marketed batteries comprising a hundred series-connected cells, this means only one is used to the full, and the other ninety-nine are more or less underexploited.
To exploit all the cells to the full, electric energy management (or so-called equalizing) units have been proposed to transfer electric energy between the cells during operation of the battery. For example, as the battery discharges, the electric energy management unit transfers electric energy from the better-performing (i.e. higher-capacity) cells to the lesser-performing (i.e. lower-capacity) cells, so that all the cells are exploited to the full.
Currently marketed electric energy management (or equalizing) units, however, are complicated, bulky, and expensive, and not particularly efficient energy-wise.
WO2007145463A1 describes a management unit for managing electric energy stored in a battery comprising a number of series-connected cells, and which comprises: a main equalizing circuit, which forms the primary of an electric transformer and has an alternating-current electric generator; and, for each battery cell, a secondary equalizing circuit, which forms the secondary of the electric transformer, and is therefore coupled magnetically to the main equalizing circuit, is connected parallel to the cell, has a one-way electronic device which imposes electric current flow to the cell in one direction only, and is connected to a drive device that can be activated to zero the voltage applied to the cell by the secondary equalizing circuit. The electric generator supplying the main equalizing circuit uses the battery voltage, and comprises a CSW redistribution transistor, which is opened or closed to enable or disable energy transmission from the higher- to lower-charged cells.